Stretch forming is a method of forming parts by stretching a metal sheet beyond its elastic limit over a die. The sheet is deformed by tension forces into a shape corresponding to the die, and retains this shape because it is stretched beyond its elastic limit. This method is widely used in aircraft and automotive fields for fabricating sheet metal components, and is further described in U.S. Pat. Nos. 2,824,594, 2,835,947, 3,073,373, 3,299,688 and 3,575,031.
In conventional stretch forming presses, a metal blank, typically flat aluminum or steel sheet stock having a generally rectangular shape, is loaded into a pair of opposed gripping jaws which are then positioned by associated tensioning hydraulic cylinders to an initial stretch forming position. A die carried by a main hydraulic cylinder is driven against the blank with sufficient force that the blank is stretched beyond its elastic limit over the die. Alternatively, the jaws are further moved by the tensioning cylinders to wrap the blank over the die. The blank is thereby formed into a desired shape which corresponds to the shape of the die.
The jaw assembly of such stretch forming presses may have a wide non-segmented jaw for gripping the end of the metal blank, or may have a multiple-segmented jaw. For example, in instances where the die has a straight contour transverse to the metal blank, it is desirable that the margins of the blank be gripped by a wide non-segmented jaw or by multiple-segmented jaw in which the segments are aligned in a straight line. When the die has a curved contour transverse to the metal blank, it is desirable to grip the ends of the workpiece with a plurality of jaw segments which can be moved relative to each other to follow the contour of the die.
In U.S. Pat. No. 2,835,947 to Gray et al, a jaw assembly is disclosed having four jaw segments. The jaw segments are mounted on a pair of spaced-apart platens which are in turn mounted on a single mount. The mount is connected by a shaft to a tensioning device for moving the jaw horizontally forwardly and rearwardly. The platens can be rotated relative to the mount. Each segment is pivotally coupled to adjacent segments and can rotate and move transversely within guideways in the platens. The jaw segments can be aligned in a straight line or one which curves downward on one or both sides.
U.S. Pat. No. 3,299,688 to Gray discloses a stretch forming press having a plurality of jaw segments wherein each segment is separately connected to a hydraulic cylinder and forms an independent tensioning unit. In such an apparatus, the position of the jaw segments is independently adjustable. A cable extends through the jaw segments to loosely maintain their relation to each other.
In another known multiple segment jaw assembly, the segments are simply hinged to each other with the center segment being connected to the tensioning device by means of a yoke and a rearwardly extending shaft. In such an assembly, the widths of the segments are generally equal, however, to provide adequate strength, the length of the center segment is greater than the next adjacent segments, whose lengths are greater than the next adjacent segments and so on.
In the above-mentioned stretch forming presses, the jaws assemblies are attached to the tensioning device at positions spaced apart rearwardly of their centers of gravity, in some instances by as much as 10 feet. This creates a substantial rotational moment on the point of attachment which must be compensated for, e.g., by stop pins or the like. Any upward swing of the jaw results in loss of die table stroke. Further, movement of the jaw segments relative to each other, e.g., to form a curve, generally causes a change in the center of pull which creates an additional rotational moment about the point of attachment of the jaw assembly to the tensioning device during stretch forming operation.